(a) Field of the Invention
The present invention relates to a method and apparatus for handover, and a method of determining a delay parameter in a wireless communication system. More particularly, the present invention relates to a method and apparatus for handover between a macrocell and a small scale cell within the macrocell in a wireless communication system.
(b) Description of the Related Art
Currently, as a substitute for a macrocellular system with a radius of about 1 km, studies on small scale cellular networks such as a microcell with a radius of 50-100 m, a femtocell with a radius of 10-50 m, and a Wi-Fi hotspot have been actively performed. Such small scale cells have a merit that construction and operation cost of a base station is very low and they are generally installed in an indoor environment to provide a wireless data connection service to indoor users. Therefore, many mobile communication providers are actively pursuing construction of a hierarchical cellular network that installs a plurality of small scale cells within one macrocell radius.
Because several small base stations are simultaneously connected within one macrocell, the hierarchical cellular network can greatly increase processing quantity of a system and the number of simultaneous connections compared with an existing single macrocellular network.
However, when several small scale cells are included within the macrocell, the number handovers between the macrocell and the small-scale cell significantly increases. Accordingly, a macrocell base station that should simultaneously process handover to all small scale base stations within a macrocell area has a large burden. Therefore, it is necessary to suppress unnecessary handover through selective handover between a macrocell and a small scale cell.
Most studies on reduction and suppression of unnecessary handover until now have been focused on solving a ping-pong effect that occurs at a boundary point of two cells. The ping-pong effect indicates that a terminal continuously repeats handover with two base stations in a short time span of within several hundred milliseconds (ms) at a boundary point of two cells. The ping-pong effect occurs as signal intensity, i.e., signal strength, frequently changes at a boundary point of two cells. Therefore, for a hierarchical cellular network environment, studies for solving such a ping-pong effect have appeared. However, in a hierarchical cellular network environment, reduction of unnecessary handover is not sufficient with only a reduction of the ping-pong effect.
Because a small scale cell has a small radius, a ratio of a temporary user that uses a corresponding cell for a short time within several seconds and that again exits the cell is very high. According to a use pattern measurement result of a microcellular system that is disclosed in a document “Mobility Patterns in Microcellular Wireless Networks”, users in which a connection time to a base station is 3 seconds or less make up 50% of all users and users in which a connection time to a base station is 10 seconds or less make up 70% of all users. This result shows that it is necessary to suppress a temporary user that is connected to a small scale cell within several seconds in addition to a ping-pong effect.
A representative study has been performed to prevent handover of such temporary users. By measuring a signal measurement result of a small scale cell base station and a moving speed and a direction vector of a user terminal, when it is determined that a user temporally stays in a cell, there is a method of not performing handover. Another method calculates a handover permission probability based on a measurement result of handover that moves from a macrocell to a small scale cell and of performing handover to correspond to a corresponding handover permission probability. However, these studies have a limitation of assumption that terminal users move in a uniform linear direction. Therefore, when applying a method of a corresponding study to an actual environment having complicated user mobility, it is expected that performance deterioration cannot be avoided.
In an existing handover technique, when a user terminal that is connected to a macrocell approaches a small scale cell base station and signal intensity of the small scale cell exceeds a given threshold, handover from the macrocell to the small scale cell is immediately performed. That is, in an existing handover technique, no mechanism that can prevent a temporary user from connecting is provided.